Hans Joachim Schellnhuber is the German government’s climate protection adviser and a distinguished physicist. He was interviewed by the German magazine Der Spiegel last week and gave a starkly simple view of how much CO2 the world can emit.

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To achieve a two-in-three chance of keeping temperature increases below 2 degrees, humanity can only emit about 750 billion tonnes of CO2 between now and 2050, he said. (I assume that after 2050 Schellnhuber believes that net emissions must fall to zero or below.) 750 billion tonnes spread over today’s world population of 6.7bn means about 110 tonnes per person between now and mid-century.

It sounds a lot. But European countries today emit about 11 tonnes annually for each inhabitant. It would be more if we counted the embedded emissions in goods we buy from China and elsewhere. Using Schellnhuber’s rough numbers, Europeans will exhaust their allowance within ten years if emissions do not fall.

Put another way, to meet the target Europe’s countries need to reduce per person emissions by about half a tonne a year every year for the next twenty years. Achieving this would require a near 5% cut next year, rising to 10% a year by 2020 and 20% annual reduction by 2025. As Stern has pointed out (Stern Review, p. 231) the only time even a 5% emissions reduction has ever been achieved over a large country for several years was the 1989 to 1998 period in the former Soviet Union, when emissions fell 5.2% per year.

The 110 tonnes figure for the maximum permitted emissions per person omits consideration of the probable 40% increase in world population by 2050. This would cut the figure to 85 tonnes or so – less than eight years of current European per capita emissions. It also assumes that all world citizens have an equal right to a certain volume of emissions even though the developed world has been responsible for more than 80% of the increase in CO2 in the last century. Schellnhuber points out that if we allocated emissions rights in inverse proportion to historical CO2 output the developed countries would already be ‘carbon insolvent’.

He concludes by saying that the richest one sixth of the world should pay $100 a year per person to help reduce the future emissions of low income countries. In effect, we would be paying poor states to hold their cumulative emissions per head below 110 tonnes so that we can overshoot our allowance. Compensating the least polluting countries for the almost certain failure of the developed world to keep within our quota must surely be the broad outline of any Copenhagen scheme that will be acceptable to the global south?